The phenomenon of stick-slip for generating sound has been a part of the human experience for hundreds of years. When it is properly controlled the result is music as experienced when a musician draws a horse hair bow coated with rosin across a violin string. If, however, the musician presses too hard on the bow and reduces the velocity of traverse across the string, the result is a nerve jangling squawk. A fingernail drawn across a blackboard gives the same effect as does the pressure plate of a clutch at the moment when the non-rotating surface of the clutch throw-out bearing contacts the rotating release fingers of the clutch pressure plate. The fingers of the clutch pressure plate act as tuning forks, which, in conjunction with the spring steel pressure plate, act as a sounding board vibrating as does a cymbal. Together they make a dissonant combination of tones that cannot be ignored and are very objectional.
Prior art has treated the problem of noise in a clutch by introducing various devices in the form of resilient linings and torsional vibration dampeners in proximity with the clutch friction plates. Nowhere, however, has prior art addressed the problem of axial vibrations which originate with the clutch pressure-plate release fingers. This undoubtedly is due to the difficulty encountered in isolating vibration sources in an assembly as complex as an operating clutch. In the case of the present invention, the source of vibrations was discovered while rubbing the clutch release fingers with an alcohol saturated cloth to remove grease. A characteristic sound was produced that was immediately identified with the chirp that is produced when the non-rotating clutch throw-out bearing contacts the relatively rotating clutch release fingers. Armed with this information, attention was focused on possible methods of preventing resonance in the release fingers.
Two courses of action were considered. First, the elimination of the stick-slip condition that prompts the resonance and second, dampening or altering the vibrations induced by stick-slip so that they do not become objectionable. Elimination of stick-slip by treatment of contacting surfaces was given second priority because it was felt that any such treatment would have little permanency in the environment of an operating clutch due to the presence of wear and wear particles. It was reasoned that wear particles could indeed become the equivalent of rosin on a violin bow which enhances the generation of stick-slip.